1. Field of the Invention
This invention relates generally to an improved type of crane and, more specifically, to tension supported boom cranes which utilize one or more boom support lines to restrain the load sheave supporting boom.
2. Description of the Prior Art
In the past, cranes have been used extensively for lifting and, in particular, for structural erection. The mobile crane is particularly useful in this context. Mobile cranes commonly utilize a lattice type boom which is fabricated from light-weight members and functions as a compression member with a boom pivot at its lower end and a load sheave at its outer end, and is restrained by one or more boom support lines attached in tension at a single given distance from the boom pivot. Another common feature of a mobile crane is that, in addition to a main boom, an auxiliary boom is swung into place and locked, or otherwise attached to, the end of the main boom, providing an additional load sheave at a point more remote from the boom pivot thereby providing an increased reach at a reduced capacity. The load line is passed over the load sheave and singly or in combination with other parts of line lifts a load block when the load line is wound onto the load hoist drum.
A problem common to all cranes utilizing a single load block is that the attitude of the load is dependent upon the load block being attached directly to the center of gravity of the load. Failure to position the load block directly over the center of gravity will result in an inclined attitude of the load which may contribute to the load tilting, sliding, or even capsizing.
A further problem arises when a load is suspended from a single block in that, when the center of gravity is positioned under the single block, the load is in a state of balance and is, therefore, subject to unwanted movement from such forces as the wind.
A need existed for a boom type crane capable of supporting a load at a plurality of points along the length of the boom.